This invention relates to carriers for wafers that are processed into semiconductor units, more particularly, the invention relates to sealable enclosures for holding wafers in a horizontal orientation.
Semiconductor wafers are subjected to numerous steps during processing in various pieces of processing equipment. The wafers must be transported from workstation to workstation and often must be temporarily stored in order to accommodate the necessary processing steps. Usually such transport and storage is accomplished by putting the wafers into sealed containers to minimize exposure of the wafers to environmental contaminants.
As semiconductors have become larger in scale, that is, as the number of circuits per unit area has increased, contaminants in the form particulates have become more of an issue. The size of particulates that can destroy a circuit has decreased and is approaching the molecular level. Particulate control is necessary during all phases of manufacturing, processing, transporting, and storage of semiconductor wafers. The industry is moving toward processing larger and larger wafers into semiconductors. Three hundred millimeter (300 mm) wafers are now commonplace.
Numerous configurations of carriers have been previously known for handling, storing, and shipping wafers. A common component in carriers configured as closable containers is a molded plastic member known as a cushion to stabilize the positioning of wafers in the container. Such shipping containers with cushions have been previously known as disclosed, for example, in U.S. Pat. Nos. 4,043,451; 4,248,346; 4,555,024; 5,253,755; 5,273,159 and 5,586,658. These types of containers typically include vertical wafer-receiving channels and cushions at the upper and/or lower ends. Various configurations of cushions have been utilized for securing the vertically oriented wafers in the channels. For example, the cushions may comprise fingers that extend from or attach to the cover for gripping the wafer edges. These cushions conventionally have a wafer engaging portion with a V-shaped cross section. An integral finger portion connects to the wafer engaging portion and also connects to a base attached to the cover.
Such conventional cushions are manufactured from relatively pliable plastic such as polyethelene, or even more pliable plastics, to minimize the pressure exerted on each wafer edge. Such materials are subject to creep and the loss of resiliency and cushioning performance. These cushions may be integral with the container closure (see for example U.S. Pat. No. 5,586,658 assigned to owner of the instant invention) or may be attached to the closure by various means such as a snap-in arrangement (see U.S. Pat. No. 4,880,116 to Kos). Highly stable and precise attachment of cushions to door have been elusive. Metallic fasteners, due to their potential of creating metal particulars are to be avoided.
These shipping devices have typically been designed to transport wafers or disks in a vertical orientation from place to place, whereas most processing workstations require that wafer processing carriers retain wafers horizontally. Thus wafers must be reoriented for many processing steps. Where the entire container is reoriented with wafers in place, the wafers can shift and scrape against the wafer pockets producing particulates.
Corresponding with the increase in the size of wafers being processed into semiconductor chips, the industry is also shifting from vertically oriented containers to containers that maintain the wafers horizontally. Forward wafer restraints for such carriers have comprised a cushion member attached to the door and extending vertically and transverse to the planes of the wafers to engage each wafer along a vertical line. The cushions may be fixed to the inside surface of the door such that they engage the stack of wafers as the door is inserted into the door frame or alternatively, the cushions may be attached to mechanisms in the door to extend and retract the cushions independently of the movement of the door into and out of the door frame. Known cushions for such horizontal containers consist of a vertically positioned resilient rod or bar or a vertically oriented elongate base member with a plurality of fingers extending therefrom to engage the edge of each sequential wafer. See U.S. Pat. No. 5,711,427 which is incorporated herein by reference.
With the vertically oriented carrier, wafer receiving channels are designed to hold wafers firmly with a minimum of horizontal movement. With the horizontally oriented carrier, wafer receiving slots are larger than the thickness of wafers to enable wafers to be inserted horizontally into containers and lowered onto a seating position on wafer shelves. To avoid particulate generation, there will ideally be no sliding of the wafer on the wafer shelves. In such carriers, now known as transport modules, contact with the wafers by the carrier is desirably kept to a minimum. For example, as disclosed in U.S. Pat. No. 5,788,082, which is incorporated herein by reference, see FIGS. 14, 16, and 17 and the text associated therewith. Minimal contact with the wafers is believed to create minimal opportunity for particulate generation and particulate contamination of the wafers. The shelves and integral beads which provide the minimal contact as disclosed in U.S. Pat. No. 5,788,082, are made of specialized abrasion resistant materials. Conventional cushion finger (and other cushion designs) for engaging wafer edges grip the wafer edge and restrain the wafer in both axial directions and radially. Such conventional fingers contact the circumferential surface of the wafer as well as providing the constraining forces at the edge between planar top and bottom surfaces of the wafer and said circumferential surface. Such gripping is inimical to the minimal contact with the wafer; and in transport modules, the need for axial (up-down) constraint of the wafers by the cushions is not necessary in that the modules are typically utilized for transporting between workstations or for temporary storage of the wafers. Thus severe jostling of the carriers, such as may occur with shipment from facility to facility, is typically not a consideration with transport modules.
The 300 mm wafers are significantly heavier than previous 200 mm and smaller wafers. Although this weight operates to effectively maintain the wafers seated on the wafer shelves, the weight also renders traditional resilient cushioning fingers, such as made from polyethelene, ineffective to maintain the radial position of the wafers on the shelves. Any sliding of the wafer on the shelves has the potential of creating damaging particulates. Thus a more rigid cushion is needed beyond conventional polyethelene. Use of stiffer materials which would have a correspondingly greater spring constant would require greater precision in manufacturing. Deflection of the fingers would need to be less and more carefully controlled to avoid putting excessive force on the edge of the wafers subjecting them to risk of damage. Moreover, the cushioning members would need to be securely anchored to the closure to maintain the controlled deflection and controlled force on the wafers. These requirements are difficult to accomplish in the sizes associated with 300 mm transport modules.
Conventional individual resilient fingers for engaging wafers, whether for horizontal or vertically oriented wafers, have a configuration of a V-shape with a bottom wafer seating portion that is sized to the thickness of the wafer to hold the same securely. Moreover, each wafer is supported at the same circumferential position. That is, the cushions are arranged in a linear row or column. The spacing between horizontal wafers in 300 mm front opening carriers has been standardized by the industry to allow maximum density in the carriers while still allowing insertion room for a robotic arm for insertion and removal. This minimal amount of vertical space makes it extremely difficult to manufacture cushions with vertically aligned wafer engaging fingers and wafer engaging portions that have any allowance for slight vertical misalignments in the wafer engaging portions such as might be caused by variation in manufacture of the cushions or wafers misaligned on the shelves. Thus, a wafer cushion system for providing a forward constraint for sealable wafer enclosures is needed that provides a precise control of wafer engagement, that is stiffer than traditional cushions, that provides for manufacturing variations, wafer-cushion engagement portion misalignments, and that has minimal contact with the wafers.